Many systems rely upon the transmission of signals between devices or components. The transmission of signals may take place in single-ended mode, where a signal is transmitted over a single wire or line, or in differential mode, where a signal is transmitted over a first wire or line, and the signal's complement is transmitted over a paired wire or line. For single-ended signaling, the single transmission line can be considered the physical channel, while for differential signal, the transmission line pair can be considered the physical channel.
As is understood, differential signaling confers benefits in terms of signal integrity, including noise immunity, common mode noise rejection, and reduced electromagnetic emissions as compared to single-ended signaling. These benefits may be desired for improved transmission in certain electrically noisy environments or where the signal itself may cause interference elsewhere. However, one disadvantage of differential signaling as compared to single-ended signaling is the requirement for twice as many wires or physical transmission media (assuming that a ground wire, plane, or shield is not required, or that a single ground return can be used for multiple signal wires).
Some systems have constraints on the number of wires or physical channels available to transmit signals. For purposes of simplicity in description, the expression “physical channel” will be used to refer to any electrical conductor, as well as wireless transmission, or optical signal channel. A differential link or channel uses two physical wire or lines, and thus can refer to a pair of wires, printed circuit board traces, or other pair of conductors. The constraints of some systems mean that there are not as many physical lines available for signaling as there are signals to be transmitted. Especially when differential signaling is employed, the available wires or physical lines may be already fully occupied for a specific application without sufficient wires or physical lines for all the signals required.
Previous work, for example, as described in U.S. Pat. No. 6,492,984, recognizes that a “virtual” common-mode signal may be transmitted over an existing differential physical channel while allowing recovery of both the differential signal and the common-mode signal. However, the resulting transmitted common-mode “virtual” signals are subject to the signal integrity limitations described earlier.
Additionally, using differential signaling in some systems may require adapting certain physical interfaces to provide differential signaling. Such adaptations may prevent the use of legacy devices that are not compatible with the differential signaling interface.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.